Gas turbine engines are internal combustion engines typically used to provide thrust to an aircraft or to provide power for land-based operations. A turbofan engine is a type of gas turbine engine consisting of a fan section, and a core engine located axially downstream of the fan section and including a compressor section (including a low pressure compressor and a high pressure compressor), one or more combustors, and a turbine section (including a high pressure turbine and a low pressure turbine). Air may be drawn into the engine and accelerated by the rotating blades of the fan, and it may be subsequently routed through the core engine where it may be compressed/pressurized in the compressor section prior to entry into the combustor(s). Once in the combustor(s), the air may be mixed with fuel and combusted to generate hot combustion gases. Energy may be extracted from the combustion gas products in the turbine section to power the rotation of the turbine section. The turbine section may then, in turn, drive the rotation of the compressor section and the fan, as all may be interconnected on one or more shafts. The combustion gas products may then be expelled through an exhaust nozzle to provide aircraft propulsion.
In a two-spool turbofan engine, an inner shaft may connect the fan and the low pressure compressor to the low pressure turbine, and an outer concentric shaft may connect the high pressure compressor to the high pressure turbine. In a geared turbofan engine, the inner shaft may drive the fan at a lower speed than the low pressure compressor and the low pressure turbine through a speed changing device (e.g., a reduction gearbox) to improve engine efficiency.
Some gas turbine engines, such as geared turbofan engines, use aluminum or aluminum alloy based fan blades. The aluminum/aluminum alloy fan blades may have a polyurethane coating to protect the body of the airfoil against corrosion or erosion, as well as a wear-resistant anodized aluminum hard coating applied to the tip of the airfoil to prevent wear of the fan blade tip upon impact with an abradable seal lining the inner surface of the fan case.
Gas turbine engines may be initially assembled with a clearance between the tips of the aluminum/aluminum alloy fan blades and the abradable seal. However, during engine initial break-in, thermal or mechanical forces (e.g., centrifugal forces, etc.) may act on the fan blades causing them to expand, possibly causing the tips of the fan blades to impact or abrade away the abradable seal in an instantaneous “rub event”. A rub event may also occur, for example, during take-off, landing, or maneuvering (i.e., sharp turns). Friction-induced heat may be generated at the rub interface between the fan blade tip(s) and the abradable material during a rub event. Given the relatively high thermal conductivities of the aluminum oxide hard coating (about 30-40 watt per meter kelvin) and the aluminum fan blade (about 160 watt per meter kelvin) compared to the abradable seal (may be less than 0.1 watt per meter kelvin), much of the friction-induced heat may be conducted to the hard coating and through the body of the fan blade rather than the abradable seal, causing the temperature of the body of the fan blade to rise. In some cases, the temperature rise through the body of the fan blade may exceed the temperature capability of the polyurethane coating and lead to the melting or delamination of the polyurethane coating.
In an effort to reduce blade tip wear in aluminum fan blades due to rub events, U.S. Patent Application Publication Number 2014/0010663 describes the application of a hard crystalline oxidation layer to the tip of aluminum fan blades. The harder tip disclosed therein was reported to more easily cut into the abradable material, thereby generating less heat as a result of rub events. While effective, additional enhancements that resist heat transfer to aluminum fan blades in gas turbine engines are still wanting.
Clearly, there is a need for systems that reduce heat conduction to aluminum fan blades in gas turbine engines.